Scientists have discovered a mysterious population of previously unknown cells lurking in the human body, according to a new study.

The enigmatic new cell type, called immune cell X, is a change that can function as two other cell types. And this fake hybrid cell can trigger type 1 diabetes .

Scientists have long believed that hybrid cells such as these could not exist. The population of these cells is probably small; maybe less than seven out of every 10,000 white blood cells, says study co-author Abdel-Rahim A. Hamad, associate professor of pathology at Johns Hopkins University School of Medicine in Baltimore.

But they can play a big role in the development of autoimmunity.

"They are very rare, but we think they are very powerful," Hamad told Live Science.

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Immune Cascade

Normally, white blood cells patrol the body and swallow foreign invaders like bacteria or viruses. After melting these invaders into small fragments, the white blood cells show one of these molten fragments, or the antigen, on its cell surface.

Two types of white blood cells – B and T cells – are incredibly powerful tools in the immune system's arsenal. B cells secrete billions of individualized antibodies, which uniquely bind to specific antigens. Killer T cells destroy cells that display specific antigens, and both B and T cells can activate each other and create a powerful feedback loop.

The key to all these processes is a special protein found on the outer surface of immune cells, called major histocompatibility complex (MHC), which is where antigens dock. B cells bind the antigen to MHC, and receptors on T cells then bind to this MHC antigen complex and activate the T cells.

In autoimmune diseases, this process is damaged, and the body's own cells become inaccurate to alien enemies. In type 1 diabetes, the immune system relentlessly destroys beta cells in the pancreas that make the hormone insulin. Without insulin, the body cannot use the energy in the food and a person will eventually die.

But what initially triggers this attack, and how does it become unstoppable? Previous work suggested that the body somehow perceives the insulin molecule as foreign and triggers type 1 diabetes. And people at high risk for the disease often make slightly different versions of MHC proteins.

But it was a puzzle, because insulin did not bind well to these high-risk versions of MHC, which means that it should theoretically not trigger a strong autoimmune attack, Hamad said.

Powerful immune trigger

In the new study, published in the journal Cell, Hamad and his colleagues found a potential culprit. Using blood samples from both healthy people and those with type 1 diabetes as well as data simulations of B and T cells, the team found the presence of an "X cell" that has receptors for both B cells and T cells.

The X cell appears to be an ultra-feeding driver of autoimmunity. The X cell extracted antibodies that strongly activated T cells in samples from individuals at high risk MHC. This triggered T cells that are founded to attack pancreatic beta cells to begin dividing like crazy, Hamad said.

The team found evidence of these so-called "self-reactive" X cells in blood samples from those with type 1 diabetes, but not in healthy controls.

Not the whole story

The results are interesting, but may not be the whole story, because there are clear environmental triggers for type 1 diabetes, said Matthias von Herrath, director of the Center for Type 1 Diabetes Research at the La Jolla Institute for Immunology , who did not participate in the study.

What's more, "We don't know if there is a single specific cell manager," von Herrath told Live Science.

To really strengthen the case of the X cells, follow-up studies should try to find these fake hybrid cells in a specific body part that is linked to the disease – such as the pancreas or the lymph nodes von added. Scientists would also like to know how many of them are lurking in the body, he said.

And there is another possibility: These X cells look like both B and T cells because they are actually these two cells, and not a new, ultra-powerful cell type at all.

Scientists would like to double-check that this X-cell – which acts as a cell with two roles – is not really two "cells in a tight embrace," von Herrath said.

That said, the study was conducted carefully, with lots of tests to make sure it was not, he said.

Although the fake hybrid cell is not the whole story behind type 1 diabetes, "even finding a cell of a certain type that in some cases would be the primary obligation would be a major step forward," von Herrath said.